Atrioventricular block is a delay in the conduction of electrical current as it passes through the atrioventricular conduction system.

Atrioventricular block is classified as ,

First-degree: Electrical conduction to the ventricles is delayed

Second-degree: Electrical conduction is intermittently blocked

Third-degree (complete): Electrical conduction is completely blocked

Most types of atrioventricular block are more common among older people. The most common causes are fibrous tissue developing in the heart's conduction system and coronary artery disease. However, a few cases of heart block are caused by drugs, particularly those that slow conduction of electrical impulses through the atrioventricular node (such as beta-blockers, diltiazem, verapamil, digoxin, and amiodarone), rheumatic heart disease, or sarcoidosis that affects the heart.

Tracing the Heart’s Electrical Pathway

The sinoatrial (sinus) node (1) initiates an electrical impulse that flows through the right and left atria (2), making them contract. When the electrical impulse reaches the atrioventricular node (3), it is delayed slightly. The impulse then travels down the bundle of His (4), which divides into the right bundle branch for the right ventricle (5) and the left bundle branch for the left ventricle (5). The impulse then spreads through the ventricles, making them contract.

First-degree atrioventricular block

In first-degree atrioventricular block, every electrical impulse from the atria reaches the ventricles, but each is slowed for a fraction of a second as it moves through the atrioventricular node. First-degree atrioventricular block is common among well-trained athletes, teenagers, young adults, and people with a highly active vagus nerve. This disorder rarely causes symptoms.

ECG: Reading the waves

An electrocardiogram (ECG) represents the electrical current moving through the heart during a heartbeat. The current's movement is divided into parts, and each part is given an alphabetic designation in the ECG.

Each heartbeat begins with an impulse from the heart's pacemaker (sinus or sinoatrial node). This impulse activates the upper chambers of the heart (atria). The P wave represents activation of the atria.

Next, the electrical current flows down to the lower chambers of the heart (ventricles). The QRS complex represents activation of the ventricles.

The electrical current then spreads back over the ventricles in the opposite direction. This activity is called the recovery wave, which is represented by the T wave.

Many kinds of abnormalities can often be seen on an ECG. They include a previous heart attack (myocardial infarction), an abnormal heart rhythm (arrhythmia), an inadequate supply of blood and oxygen to the heart (ischemia), and excessive thickening (hypertrophy) of the heart's muscular walls.

Certain abnormalities seen on an ECG can also suggest bulges (aneurysms) that develop in weak areas of the heart's walls. Aneurysms may result from a heart attack. If the rhythm is abnormal (too fast, too slow, or irregular), the ECG may also indicate where in the heart the abnormal rhythm starts. Such information helps doctors begin to determine the cause.

First-degree Atrioventricular Block

For first-degree block, conduction between the atrium (P) and ventricle (QRS) is slowed without skipped beats.

Second-degree atrioventricular block

In second-degree atrioventricular block, only some electrical impulses reach the ventricles. The heart may beat slowly, irregularly, or both. Some forms of second-degree atrioventricular block progress to third-degree atrioventricular block.

Third-degree atrioventricular block

In third-degree atrioventricular block, no impulses from the atria reach the ventricles, and the ventricular rate and rhythm are controlled by the atrioventricular node, bundle of His, or the ventricles themselves. These substitute pacemakers are slower than the heart’s normal pacemaker (sinus or sinoatrial node) and are often irregular and unreliable. Thus, the ventricles beat very slowly—less than 50 beats per minute and sometimes as slowly as 30 beats per minute. Third-degree atrioventricular block is a serious arrhythmia that can affect the heart’s pumping ability. Fatigue, dizziness, and fainting are common. When the ventricles beat faster than 40 beats per minute, symptoms are less severe.

Third-degree Atrioventricular Block.

In third-degree atrioventricular block, the atria (p) beat independently of the ventricles (QRS).

Diagnosis

Electrocardiography

Electrocardiography (ECG) is used to detect atrioventricular block. Each degree of block produces a particular pattern. First-degree atrioventricular block can be detected only by (ECG), which shows the conduction delay.

Treatment

Sometimes insertion of an artificial pacemaker

First-degree atrioventricular block generally requires no treatment.

Some people with second-degree atrioventricular block require an artificial pacemaker. Almost all people with third-degree atrioventricular block require an artificial pacemaker.

A temporary pacemaker may be used in an emergency until a permanent one can be implanted. Most people need an artificial pacemaker (see Figure: Keeping the Beat: Artificial Pacemakers) for the rest of their lives, although heart rhythm may return to normal if the cause of the atrioventricular block resolves—for example, after the drug that caused the atrioventricular block is stopped or after recovery from a heart attack.

Merck and the Merck Manuals

Merck & Co., Inc., Kenilworth, NJ, USA is a global healthcare leader working to help the world be well. From developing new therapies that treat and prevent disease to helping people in need, we are committed to improving health and well-being around the world. The Merck Manual was first published in 1899 as a service to the community. The legacy of this great resource continues as the Merck Manual in the US and Canada and the MSD Manual outside of North America. Learn more about our commitment to Global Medical Knowledge.